Relax addressable requirement

Trying to use dm-core HEAD failed for me because:

```
Bundler could not find compatible versions for gem "addressable":
  In Gemfile:
    dm-core (>= 0) ruby depends on
      addressable (~> 2.2.6) ruby

    launchy (~> 2.4.2) ruby depends on
      addressable (2.3.5)
```
This PR relaxes the addressable requirement.  Before and after this change I was getting 3 spec failures.